The present invention relates to the field of plasma processing of silicon wafers and more particularly to plasma processing equipment having automated electrode handling capabilities.
As is known in the art, a fundamental step in the manufacturing of semiconductor devices, such as integrated circuits (ICs), is the process of forming electrical interconnections. The formation of electrical circuits containing components such as semiconductor transistors, involves a series of steps starting with the formation of a blank silicon wafer. The blank silicon wafer or substrate is then processed using successive steps of depositing to and etching away various materials to form the proper interconnections and therefore the electrical circuits.
Such depositing and etching operations may be performed in a plasma reactor system. In semiconductor manufacturing, plasma reactor systems are used to remove or deposit material to or from a workpiece (e.g., semiconductor substrate) in the process of making integrated circuit (IC) devices. A key factor in obtaining the highest yield and overall quality of ICs is the uniformity of the etching and deposition processes.
When it is desired to deposit materials onto the wafer, a plasma reactor system may be used to sputter a variety of materials, one of which could be silicon, onto semiconductor wafers. In these sputtering applications, a silicon disk, or silicon dioxide disk or doped-silicon disk is used as a facing on the metal drive electrode to provide a source of material to be deposited on the semiconductor wafers to form variety of circuit patterns. This silicon disk is herein referred to as the source electrode or target.
There are several different kinds of plasma processes used during wafer processing. These processes include (1) plasma etching, (2) plasma deposition, (3) plasma assisted photo resist stripping and (4) in situ plasma chamber cleaning. One artifact of these plasma processes is the erosion of the source electrode as it is consumed during the formation of plasma, hence, a purpose for the source electrode is to serve as a protective barrier between the driven RF electrode and the plasma. Furthermore, each of these plasma processes has associated plasma density non-uniformities, for example, due to the generation of harmonics of the plasma excitation frequency. These non-uniform plasmas erode the plasma system silicon source electrode non-uniformly. The non-uniformly etched silicon electrode in turn exacerbates the non-uniformity of the plasma. To ensure uniform plasmas, these silicon electrodes are changed frequently. Otherwise, if a system with a non-uniform plasma is used for semiconductor wafer processing, the non-uniform plasma discharge can produce non-uniform etching or deposition on the surface of the semiconductor wafers. Thus, the control of the uniform etching or erosion of the silicon electrode directly affects the quality of integrated semiconductor chips manufactured by the semiconductor industry.
As illustrated in FIG. 1, a typical prior art plasma reactor system 10 includes, a plasma chamber 11 in which a wafer 18 is processed. Wafer 18 is placed on a chuck 16 and exposed to various plasmas depending on whether the wafer is undergoing an etch or deposition step. The plasma formed in plasma chamber 11 also varies depending on the material being deposited or etched on wafer 18. The plasma within chamber 11 is formed by electro-mechanically coupling a source electrode 14 to the metal drive electrode 12 and driving a RF signal through the metal electrode 12 and consequently through source electrode 14. Source electrode 14, in effect, becomes the electrode in direct physical and electrical contact with plasma. The plasma formed within chamber 11 depends on a variety of factors including the RF power magnitude, the RF drive frequency, the chamber gas pressure and the composition of gases residing in the chamber. As described above, during processing of silicon wafers, a silicon electrode may be used as the source electrode.
As also described above, during silicon wafer processing, the silicon electrode is consumed and thus must be changed periodically in order to maintain consistent processing conditions within the plasma chamber. In prior art systems such as that shown in FIG. 1, the silicon source electrode 14 is attached to the metal electrode 12 by means of metal screws 23 which pass through clearance holes in the silicon electrode and mate with threaded holes in the metal electrode or metal nuts 25 on the back side of metal drive electrode 12. The clearance holes in the silicon electrode are countersunk to assure that the heads of the attachment screws do not protrude beyond the surface of the silicon electrode.
Due to the electrical, thermal and physical contact requirements between the silicon disk and the drive electrode, there is a need to insure proper electrical and mechanical connection between the silicon electrode 14 and the metal electrode 12. Even when an initial proper contact is established between metal drive electrode 12 and silicon source electrode 14, as plasma processing proceeds and the silicon drive electrode is consumed, the plasma changes. However, systems currently available, such as plasma system 10 shown in FIG. 1, provide no means of adjusting the source electrode/metal drive electrode contact during processing. There is therefore a need to provide a way to adjust the contact of the silicon disk to the electrode in real time during a wafer processing step and/or between wafer processing steps in order to fine-tune plasma uniformity.
Furthermore, since the silicon source electrode described above is consumed during use, it must be changed on a relatively frequent basis. Even when the silicon source electrode is not consumed, it may be desirable to change a source electrode to one made of a different material or one having a different shape to produce a different plasma in plasma chamber 11. Unfortunately, in prior art plasma processing systems, changing the source electrode 14 requires shutting down the plasma process, venting the chamber, opening the chamber, removing the attachment screws, replacing the consumed silicon electrode manually with a new one and putting everything back together again. This process is time consuming, reduces wafer throughput, and may create added defects through contamination.
It would be advantageous therefore to provide a plasma processing system where the source electrode could be replaced automatically without opening the plasma chamber. It would be further advantageous to provide an apparatus for plasma processing of semiconductor wafers which allows for secure, repeatable attachment of a source electrode to the metal drive electrode where the contact between the source electrode and metal drive electrode is dynamically adjustable during operation of the plasma processing system.
According to one aspect of the present invention, a plasma processing system is provided that includes an automated electrode retention mechanism as well as an automated electrode handling system. The retention mechanism includes an elevator system that raises and lowers a source electrode in order to dynamically couple and decouple the source electrode from the drive electrode. In one embodiment the automated retention mechanism includes a plurality of lift arms coupled to associated drive units. Activating the drive units causes the lift arms to move in a vertical direction within the plasma processing system vacuum chamber. Coupled to the lower end of the lift arms is an electrode shelf that supports the source electrode while it is being raised and lowered as well as retaining the source electrode to the drive electrode. The shelf is generally ring-shaped and supports the source electrode about the perimeter of its lower face. That is, the interior diameter of the shelf is slightly less than the diameter of the source electrode. In the preferred embodiment, the shelf is made of quartz.
According to another aspect of the present invention, in addition to providing the ability to raise and lower the source electrode, the retention mechanism also functions to provide dynamic control of the contact force between the source electrode and the drive electrode. With such an arrangement, a change in the contact forces can be made to eliminate plasma nonuniformities associated with a slowly eroding electrode.
According to yet another aspect of the present invention, an automated electrode handling system is provided that allows a source electrode to be changed in a plasma processing system without the need to dismantle the system. The automated electrode handling system includes a robotic arm that is capable of retrieving a used source electrode from the automated electrode retention mechanism and placing it in a discard station within the system. Additionally, the automated electrode handling system is capable of retrieving a new source electrode from a staging area within the plasma processing system and placing it on the automated electrode retention mechanism such that it can be brought into contact with the drive electrode. Preferably, the automated electrode handling system is configured to work in conjunction with a load-lock system in order to isolate the working mechanisms of an automated electrode handling system from the plasma processing vacuum chamber during processing of wafers. With such an arrangement, a source electrode can be replaced in a plasma processing system without having to fully shut down the system or dismantle the system. As such workflow throughput in a plasma processing system so equipped dramatically increases.